1. Field of the Invention
This invention relates to a method for detecting artifacts in data, more particularly, data observed during physiological testing.
2. Discussion of the Art
U.S. Pat. No. 6,526,298 discloses an apparatus and method for measuring the concentration of an analyte (such as glucose) within a biological tissue. The method involves the steps of controlling the temperature of the tissue, introducing electromagnetic radiation into the tissue, and measuring the intensity of the electromagnetic radiation emitted from the surface of the tissue. The period of contact between the apparatus and the tissue lasts several minutes. Optical data from a plurality of channels of the apparatus are recorded at regular time intervals during each measurement. See FIGS. 1A, 1B, 1C, and 1D, which show various trends of data. The optical data, which are converted to voltage levels by a photodetector, are used to determine the concentration of an analyte, based on empirical models.
A successful determination of the concentration of an analyte requires proper thermal and optical contact between an optical probe and the surface of the tissue. Example of such devices are described U.S. Pat. No. 6,241,663; U.S. Pat. No. 6,353,226; U.S. Pat. No. 6,526,298; U.S. Pat. No. 6,615,061; U.S. Pat. No. 6,662,030; U.S. Pat. No. 6,662,031; WO 99/59464; WO 2002/060320A1; WO 2002/082989A1; U.S. Ser. Nos. 09/419,461, filed Oct. 15, 1999, and 09/834,440, filed Apr. 13, 2001; and Khalil, et al., “Temperature modulation of the visible and near infrared absorption and scattering coefficients of intact human skin”, J. Biomedical Optics, 8(2), 191-205 (April 2003); Yeh, et al., “Near-infrared thermo-optical response of the localized reflectance of intact diabetic and non-diabetic human skin”, J. Biomedical Optics, 8(3), 534-544 (July 2003); and Yeh, et al., “Monitoring Blood Glucose Changes in Cutaneous Tissue by Temperature-modulated Localized Reflectance Measurements”, Clinical Chemistry 49:6 924-934 (2003).
The preferred optical probe is a combination optical/thermal head that contacts the tissue. The probe preferably comprises a thermally conductive temperature-controlled disc, preferably made of aluminum, and a fiber-optic bundle at the center of the disc. Failure to maintain proper contact between the probe and the surface of the tissue can result in artificial perturbations (artifacts) in some or all of the recorded data. See FIGS. 2A, 2B, 2C, and 2D, which show various artifacts occurring in trends of data. An artifact can result in an erroneous calculation of the concentration of the analyte because the artifact distorts the actual trend of the intensity of the radiation emitted over a period of time. Contact between the apparatus and the tissue can be compromised by improper application of the probe to the surface of the tissue, by accidental movement of the probe or the tissue during the measurement, and perhaps by any substances on the surface of the tissue that interfere with the collection of radiation emitted.
Clinical tests employing a prototypical dual-sensor probe on the volar forearm of 20 diabetics produced at least 37 results (out of 400 tests) having data artifacts that were deemed unacceptable for glucose modeling. Most of these artifacts were assumed to result from human error by the patient or by the clinician and cannot be totally prevented. Some patients experienced a percentage of artifacts much higher than average.
It is believed that a primary cause of data artifacts is movement of the forearm during a measurement. In a confirmatory measurement, subjects were instructed to remain motionless during each measurement, except at one-minute intervals, when they intentionally moved their forearms (in contact with the probe) in a specified manner. The movements generally coincided with the production of artifacts in the optical signals, as shown in FIGS. 3A, 3B, 3C, and 3D.
Mathematical detection of artifacts with the apparatus is difficult, because the optical data can exhibit numerous trends that are unpredictable. FIGS. 1A, 1B, 1C, and 1D depict normal data that move in an upward trend, in a downward trend, in a horizontal trend, sometimes changing direction or modulating during a test, respectively. Causes of these variations include the difference in the temperature of the probe and the initial temperature of the tissue, changes in optical properties of the tissue as the temperature of the tissue changes, variations in properties of the tissue among the population (optical, thermal, structural, etc.), and variations in the biological responses to temperature of the probe among the population. FIGS. 2A, 2B, 2C, and 2D depict how the data artifacts can differ in magnitude and direction.
Methods for identifying motion artifacts are known in the art of photoplethysmography, namely where measurements in which a periodic signal associated with the heartbeat is monitored over a long period of time, typically in recovery, intensive care, and emergency rooms. The algorithm for detection of motion artifacts depends on differentiating the periodicity of the true signal and non-periodicity of the motion-induced erratic signal. Techniques such as Fourier transform are used to assign a frequency to the periodic signals and reject the non-periodic signals. Examples of this approach are shown in U.S. Pat. Nos. 6,018,673 and 6,374,129. Another approach is the signal extraction technology described in U.S. Pat. Nos. 5,782,757; 6,002,952; 6,067,462; and 6,236,872. However, a problem arises when the detection method does not depend on the heartbeat and the sequence of data points has no inherent periodicity. This problem is particularly apparent in the case of the non-invasive determination of the concentration of glucose, or continuous monitoring of hematocrit, involving the use of methods described in U.S. Pat. No. 6,662,031; Zhang et al., “Investigation of Noninvasive in Vivo Blood Hematocrit Measurement Using NIR Reflectance Spectroscopy and Partial Least Squares Regression”, Applied Spectroscopy, Vol. 54, No. 2, 2000, pp. 294-299; and Wu et al., “Noninvasive Determination of Hemoglobin and Hematocrit Using a Temperature-Controlled Localized Reflectance Tissue Photometer”, Anal. Biochem., 287, 284-293 (2000).
It would be desirable to develop a method for detecting artifacts in order to alert the user that a measurement may be erroneous and that a repeat measurement is necessary. Such a method would be useful for preventing data points corresponding to artifacts from being included in subsequent calculations for the non-invasive determination of the concentrations of analytes, such as, for example, hemoglobin, hematocrit, and glucose.